A cathode ray tube having an evacuated envelope containing therein an electron gun for generating an electron beam, and a fluorescent surface or a target irradiated with the electron beam is used in a television receiving set, various display devices and an oscilloscope, and for recording a television picture image or the like. In such cathode ray tube, it is desirable to make substantially uniform the beam spot on the target electrode over the entire surface thereof and to reduce a halo about the beam spot for the purpose of obtaining a high quality picture image. FIG. 1 illustrates states of the beam spots on the target electrode of a conventional in-line type color television tube. As shown, a beam spot 101 at the center of the target electrode 100 has a true circular configuration, but the beam spots largely defected in the horizontal direction comprises a core 103 which is shown in black in FIG. 1. and a marginal halo portion 104. Such nonuniform beam configurations are formed due to the astigmatism and the difference in the focal distances as described later and do not form high quality pictures.
For this reason, as disclosed in Japanese Laid Open Patent Specification Nos. 85666/1979 and 85667/1979, a cathode ray tube has been proposed in which an asymmetrical lens is formed by a first grid or a second grid in the electron gun for compensating for the astigmatism caused by deflection. This measure can improve the uniformity of the beam spot over the entire surface of the target electrode, but the beam diameter at the center of the target electrode becomes larger than a case wherein a symmetrical lens system is used.
Japanese Laid Open Patent Specification No. 198832/1983 discloses an improved construction in which a front focusing electrodes stage disposed between an acceleration electrodes stage and a rear focusing electrodes stage is constituted by first to third grid electrodes, a constant focusing voltage is impressed across the first and the third grid electrodes, and a dynamic voltage which increases or decreases gradually from the constant focusing voltage as the degree of beam deflection increases, is impressed upon the second grid. This construction can obviate the astigmatism, but can not solve the problem regarding the difference in the focal distances caused by the difference in the degree of beam deflection. In order to solve the problem, an expedient has been made wherein, at the peripheral portion where the degree of deflection of the beam is large, the focusing voltage is made high to decrease the lens effect and to increase the focal length, so that the beam always focuses on the target electrode. However, this measure is complicated because of the necessary use of another dynamic voltage and a consideration to the effect of the third grid electrode which shortens the focal distance than a case not using the third grid electrode.